1. Field of the Invention
The present invention relates to an acousto-optic tunable filter, more particularly to an all-fiber acousto-optic tunable wavelength filter which is useful in optical communications or optical fiber sensor systems.
2. Description of the Prior Art
Wavelength filters have wavelength-dependent transmittance, so that they play an important role in many optical systems. Especially, as optical communications and optical fiber sensor systems are rapidly developed, wavelength filters have been actively studied as they are suitable for the selection of desired wavelength or gain flattening of optical amplifiers.
The wavelength filters used in optical communications or optical fiber sensor systems can be grouped into insertion type filter and all-fiber type filter based on the filtering device.
The insertion type filter is manufactured by connecting optical fiber to the integrated optical device which integrates filtering structure in an optical crystal or the like. An example of such a device is discussed under the article "Integrated optical acoustically tunable wavelength filter" by J. Frangen et al. in Electronics Letters 1989, Vol. 25, Iss 23, pp. 1583-1584. In the article, acousto-optic tunable filter is manufactured by forming light waveguides on a lithium niobate(LiNbO.sub.3) single crystal substrate. However, in this insertion type filter, optical fiber should be connected to integrated optical device, as there is considerable loss at the connected portion, which results in great loss for the passing signal light. Moreover, since some integrated optical devices show significant polarization dependence, the intensity of the output light after passing through the devices may change considerably according to the polarization state of the input light.
All-fiber type filters have filtering effect using the relation between modes of propagating light in optical fiber, providing low connection loss compared with the insertion type filter.
Publications relating to such all-fiber type filters include the article "Long-period fiber-grating based gain equalizers" by A. M. Vengsarkar et al. in Optics Letters, Vol. 21, No. 5, p. 336, 1996. In this article, the long-period grating filter is provided by periodically varying the refractive index of the core within single mode optical fiber. The advantages of this filter are that there is almost no connection loss or characteristic variation depending on the polarization state of input light. Another advantage of this filter is that desirable filters are manufactured by adjusting the periodicity of refractive index variation of the optical fiber core. However, once the filter is manufactured, the wavelength characteristic of the filter cannot be changed easily.
Other prior arts relating to all-fiber type filters include a device using two mode optical fiber which is disclosed by B. Y. Kim et al. in the article "All-fiber acousto-optic frequency shifter" of Optics Letters, Vol. 11, p. 389, 1996 and U.S. Pat. No. 4,832,437. This device can preferably change the central wavelength of the filter, but one problem with this device is that the wavelength characteristic of the filter is extremely limited.